Do self-watering pots and planters actually work?

The name oversells it. These are reservoir planters: they do not sense anything, they simply hold water below the compost and let it wick upwards. Understanding that makes it obvious which plants they suit and which they slowly drown.

Step by step

1. Understand the mechanism.A sealed reservoir sits under a false floor, with either a wick or a column of compost dipping into it. Water moves up by capillary action at whatever rate the compost demands. There is no valve and no sensing.
2. Match the plant to it.Excellent for tomatoes, peppers, cucumbers, salad, herbs like basil and parsley, and most summer bedding — all of which want consistently damp compost. Poor for lavender, rosemary, thyme, succulents, alpines and most shrubs, which need to dry out between waterings.
3. Water from the top for the first fortnight.A new plant's roots are all in the upper compost and cannot reach down to the wicking zone. Watering from above until roots grow down is the step most people skip, and it is why plants sometimes wilt in a full reservoir.
4. Use a compost that wicks.It needs to draw water upwards, which means a mix with some fine material in it. A very coarse, gritty or bark-heavy peat-free mix wicks poorly. Loam-based compost or a mix with a third loam works well.
5. Check the overflow works.There should be a hole at the top of the reservoir so rain cannot fill the whole pot. If it is blocked, an outdoor planter turns into a pot with no drainage during a wet week.
6. Flush it occasionally.Because water only ever moves upwards, salts from fertiliser and hard water accumulate at the compost surface rather than being washed out. Water heavily from the top every few weeks so the excess runs out of the overflow.
7. Drain it for winter.Outdoors, empty the reservoir in autumn and leave it empty. A full reservoir under a dormant plant means saturated compost through the coldest months, which is a reliable way to lose it, and the reservoir itself can split when it freezes.
8. Do not use them for seed sowing.Constantly damp compost at the surface is what damping-off fungus needs. Sow in ordinary trays watered from below on demand.
9. Check the reservoir size against the plant.A small reservoir under a large tomato plant in August lasts a couple of days, not a fortnight. The claimed intervals are usually for modest plants in mild weather.
10. Consider the cheap versions.A wick of capillary matting or a strip of old towel from a bottle of water into the compost does the same thing for nothing, and a capillary mat under a tray of pots works well on a greenhouse bench.

Tips

• The best use of these is for tomatoes and salad, where even moisture prevents split fruit, blossom end rot and bolting.
• Two weeks away in summer is realistic for a modest plant in a large-reservoir planter. A big tomato is not.
• Fill from the top occasionally rather than only through the fill tube, so salts get flushed through.

Common mistakes

• Using one for lavender, rosemary or succulents — They need to dry out between waterings. Constant capillary moisture rots the roots over a season and the plant declines slowly enough that the cause is never obvious.
• Filling the reservoir at planting and not watering from the top — The roots are not yet deep enough to reach the wicking zone, so the plant wilts in a planter full of water.
• Leaving the reservoir full over winter — Saturated compost and a dormant plant is root rot, and a frozen reservoir can split the planter.
• Assuming it removes the need to feed — It does not. Container plants still need feeding from about six weeks, and in a reservoir system the feed goes in the reservoir water.

Alternatives

• Capillary matting on a bench: A mat kept wet under a tray of pots does the same job for a whole benchful. Standard in greenhouses and very cheap.
• Bottle wick: A strip of capillary matting or old towel from a full bottle into the compost. Free, and good enough for a fortnight away.
• Drip irrigation with a timer: More reliable and more flexible than any reservoir pot, and it suits plants that want to dry out between waterings because you control the interval.
• A bigger ordinary pot with loam in the mix: Often the simpler answer. More compost volume plus loam-based compost is the standard fix for constant watering.

Questions people ask